For power metal oxide semiconductor field effect transistors (MOSFET), substrate resistance contributes to the total on-state resistance as a parasitic component. A vertical semiconductor device like a vertical double-diffused MOSFET (VDMOSFET), a trench MOSFET (TMOSFET), an insulated gate bipolar transistor (IGBT), or the like, represent a power switch. The resistance in the on-state of the switch is composed of a series connection of resistive elements.
FIG. 1A shows a cross sectional view of an VDMOSFET device employed in the conventional art. While FIG. 1B shows a corresponding series connection of resistive elements modeling the conductive path in the device in the on-state employed in the conventional art. The on-state resistance (Rds-on) comprises metal film resistance (Rm) 110, 120, junction between metal film and semiconductor (i.e. drain and source) resistance (Rj) 130, 140, channel resistance (Rc) 150, drift region resistance (Rd) 160, and substrate region resistance (Rs) 170. In the case of current low voltage power MOSFETs the substrate resistance (Rs) 170 contribution can be at least 40% of the total on-state resistance (Rds-on). Reducing the on-state resistance is beneficial to making the switch more efficient.
As semiconductor technology progresses, reduced on-state resistance becomes critical. In the conventional art, thinning the substrate by a method such as back-lapping and polishing the wafer can achieve some reduced on-state resistance. Wafer thickness is currently about 200 μm. Thinner wafers are possible. However, thin wafers can easily break during handling thereby resulting in lower manufacturing yields. It would be desirable to reduce the on-state resistance of the power switch without resulting in the poor manufacturing yields attributed to very thin wafers.